1. Field of the Invention
The present invention relates to a vibration damping bushing used for an automobile suspension mechanism, for example, and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, in an automobile suspension mechanism, for example, a vibration damping bushing has been used at a connecting portion of a suspension arm member to the vehicular body. Such a vibration damping bushing has a structure where an inner axial member and an outer cylindrical member externally fitted thereto are elastically connected by a main rubber elastic body.
The vibration damping bushing often requires hard spring characteristics in the axis-perpendicular direction for the purpose of improving the driving stability of motor vehicles. Therefore, as described in Japanese Unexamined Utility Model Publication No. JP-U-4-111933, a proposal is made that an intermediate sleeve made of metal or the like be embedded in and fixed to the middle portion of the main rubber elastic body in the radial direction.
Also, from the standpoint of improving the riding comfort of motor vehicles, the inner axial member and the outer cylindrical member are often required to have soft spring characteristics in the prizing direction inclined to each other. Now, as described in Japanese Unexamined Utility Model Publication No. JP-U-3-012029, a proposal is made to set the spring constant large enough in the axis-perpendicular direction while restricting any increase in the spring constant in the prizing direction by means of tapering off both ends of the outer cylindrical member in the axial direction by a drawing operation.
However, in the structure using the intermediate sleeve as described in the former example, it ends up with having hard spring characteristics not only in the axis-perpendicular direction but also in the prizing direction, which poses a problem of hardly being able to avoid deterioration in the riding comfort of motor vehicles.
Also, in the structure where both axial ends of the outer cylindrical member are tapered off as described in the latter example, significant pre-compressive deformation is caused due to the drawing operation of the outer cylindrical member at the axial end faces of the main rubber elastic body where the strain generated by a load input in the prizing direction is maximized. For that reason, during load input in the prizing direction, the strain and stress generated at the axial end faces of the main rubber elastic body get further increased, which tends to cause cracks therein posing a problem of hardly being able to achieve the required durability.